A virtual machine (VM) is a software implementation of a machine (i.e., a computer) that executes programs like a physical machine would. Generally, access to resources in virtualized environments is associated with a high level of overhead because the VM typically cannot directly communicate with a virtualized resource. A hypervisor that hosts the VM typically has the burden of emulating the needed interface or driver on behalf of the VM. In other words, the hypervisor presents the VM with a virtual platform and monitors the execution of the VM and how the VM accesses the available hardware resources.
For example, for a VM to read or write data to a virtualized storage device, control of the read or write operations will have to be transferred between the VM and the hypervisor. The hypervisor may use any type of storage interface for storing the VM's data, including local file system, network file system, network block device, etc. In a purely emulated case, the VM has no knowledge that the provided interface (i.e., the block device) is not a real physical device, because the hypervisor emulates the interface in software. A high overhead is associated with such emulation. The operational overhead is even more costly, if there is a need for transferring control intermittently between the VM and the hypervisor during I/O operations.